The development of Bragg grating reflectors within single mode fibers that are stable and highly selective in wavelength and the demonstration that erbium doped fiber amplifiers (EDFA) are capable of broadband amplification of signals at different, closely space wavelengths, has caused those in the art to consider fabricating a wavelength division multiplex, fiber optic transmission system. However, in order to fabricate such a transmission system, it becomes readily apparent that there is a need for a device for use in adding or dropping a light signal at a predetermined center wavelength to or from such a fiber optic transmission system without disturbing other signals at other wavelengths.
The following describes various devices in the prior art which are candidates for use in fabricating the required add/drop device. A publication entitled "All-Fibre Narrowband Reflection Gratings at 1500 nm" by R. Kashyap, J. R. Armitage, R. Wyatt, S. T. Davey, and D. L. Williams, published in Electronics Lett., vol. 26, 1990, pp. 730-732, discloses a 50%.times.50%, 2 by 2 fiber coupler which is used to couple signal input at all wavelengths and to extract Bragg reflected light. The device and its method of use are disadvantageous in that there is a loss of at least 75% of the reflected light intensity.
A publication entitled "Formation of Bragg gratings in optical fibers by a transverse holographic method" by G. Meltz, W. W. Morey, and W. H. Glenn, published in Optics Lett., vol. 14, 1989, pp. 823-825, discloses the use of a beam splitter to couple broadband light to a Bragg grating. The device and its method of use are disadvantageous in that the beam splitter attenuates both transmitted light and, even more so, reflected light.
A publication entitled "Intermodal coupler using permanently photoinduced grating in two-mode optical fibre" by H. G. Park and B. Y. Kim, published in Electronics Lett., vol. 25, 1989, pp. 1590-1591, discloses a device in which mode coupler gratings are formed by photorefraction in slightly multimode elliptical fibers. The device is disadvantageous in that the grating periodicity is very large and, as a result, the device is not suitable for use in fabricating efficient wavelength division multiplex components.
A publication entitled "Narrow-Bandwidth Optical Waveguide Transmission Filters" by K. D. Hill, D. C. Johnson, F. Bilodeau, and S. Faucher, published in Electronics Lett., vol. 23, 1987, pp. 465-466, discloses a Sagnac loop reflector device which consists of: (a) a twin core fiber at the input and output of the loop and (b) a Bragg grating in the loop which is used to isolate a narrow band of wavelengths. This device and its method of use are disadvantageous in that a beam splitter or coupler is required to transmit the other wavelengths and this causes light loss. Also, for proper functioning, optic path lengths in the loop have to be controlled to fractions of a wavelength and this is difficult to achieve.
In light of the above, there is a need in the art for a device for use in adding or dropping light signals at predetermined center wavelengths to or from a wavelength division multiplex, fiber optic transmission system which carries signals at other wavelengths, which device overcomes the above-described disadvantages. Further, there is a need for such a device for use in adding or dropping a light signal with minimum loss to the light signal being added or dropped and to light signals at other wavelengths already on the transmission system. Still further, there is a need for such a device for use in adding or dropping which can add or drop a predetermined fraction of the light signal. Yet still further, there is a need in the art for such a device for use in adding or dropping wherein the center wavelength of the light signal which is added or dropped is tunable.